1. Field of the Technology
The present technology relates to an image forming apparatus such as a printer, copier or the like, which forms images on sheets of paper, in particular, relating to an image forming apparatus with a transfer belt cleaning function for preventing an optical sensor that detects the density and misregistration of the image, from detecting shutter opening and closing in error.
In image forming apparatuses based on electrophotography, a test pattern image (reference toner image) is often used in order to check whether the image forming process is correctly carried out. Typical examples of test pattern images include the toner image for density measurement and the toner image for positional check.
This test of the image forming process is carried out by forming a reference toner image on a trial basis and sensing the density and position of the thus formed reference toner image with an optical sensor so as to detect whether a toner image having a correct density is formed at an exact position. In this test, if the conditions of the image forming process are not pertinent, the image forming process conditions are corrected in accordance with the detected result of the optical sensor. This test using a reference toner image is periodically implemented, whereby it is possible to achieve image forming operations under pertinent conditions.
In color image forming apparatuses, it is impossible to correctively reproduce a color image unless images of color separations are laid over in register with each other. For this purpose, the above-described detection, or check on whether there is any problem with toner density and image position, is periodically implemented using an optical sensor with a reference toner image.
However, the optical sensors used for toner density sensing and registration sensing have to be arranged at positions where an unfixed toner passes through. Besides, the aforementioned reference toner image is formed on the transfer belt and detected by the optical sensors, and in order to achieve reliable detection, the sensors have to be positioned at the most downstream side in the image forming process.
2. Description of the Related Art
Accordingly, quite a few amount of unfixed toner scatters when the recording medium (paper) is transferred from the toner belt to the fusing device. As a result, unfixed toner gradually builds up on the detection surface of the sensors, possibly causing deterioration in detection and detection failure.
To deal with this problem, JP2001-100597A, discloses a configuration in which a shutter is provided in the vicinity of the detecting surface of an optical sensor in an openable and closable manner so that the shutter will open to permit detection only for the time of measurement and will close itself otherwise, to thereby prevent adherence of unfixed toner. Further, the opening and closing of the shutter that protects the detecting face of the optical sensor for detecting the light reflected off the reference toner image against toner may be detected based on the reflected light from the transfer belt, whereby it is possible to check whether the shutter is correctly opened or closed without providing a separate sensor for detecting the opening and closing of the shutter.
However, in the above configuration, if a toner has adhered on the transfer belt when the shutter changes its position from the closed state (no direct light reflection received) to the open state (direct light reflection received), the optical sensor receives no direct light reflection due to the toner, and hence erroneously detects the shutter being closed even though the shutter is opened.
In the configuration described above, if, for example, a trouble has occurred in the main charger for charging the photoreceptor, the toner has adhered on the entire surface of the photoreceptor, and the toner extensively transfers to the transfer belt. When the opening and closing of the shutter is detected based on the presence/absence of reflected light, no direct light reflection cannot be detected due to the toner. As a result, even though the shutter is correctly opened and closed, the malfunction of the shutter's opening and closing mechanism is mistakenly detected. That is, it is impossible to distinguish the malfunction of the shutter from other failures (the operation failure of the main charger), and time-consuming checking process is consequently needed and/or there occurs another risk of doing repairs in a wrong manner.